From 1983 to 1985 he worked as a senior research fellow in the Estonian Open Air Museum, then continued his postgraduate studies.
His research work has concentrated on the history of Estonian Bar Association and Soviet time mentality among other topics.
[1] 2015-2020 Vahtre lead a group of historians, creating 2-volume research "History of the Estonian War of Independence" which was published 2021.
Just before graduation Vahtre was expelled from the Tartu State University because of his patriotic views and activities, so he had to complete his studies as an extern.
[2] Together with Laar and other patriots they formed a political association, which in 1992 developed into a National Coalition Party "Pro Patria" (Rahvuslik Koonderakond "Isamaa").